Layout: Em Box Height first baseline offset

The first baseline sits the em box top below the frame's top inset: the
font's BASE em box (idtp, else ideo + 1 em), else 0.88 em, for the
largest size on the first line. IDML FirstBaselineOffset="EmboxHeight"
imports and exports as itself; the footnote options list it and the
frame and footnote option commands take emboxHeight.

crates/compose/src/lib.rs

@@ -939,7 +939,7 @@ fn compose_with_db(doc: &Document, story: &Story, frames: &[FrameSpec], opts: &C
             let col = cur.split_rect(col);
             // Estimate slots for the breaker with the paragraph's base leading.
             let est_asc = base_chars.size * 0.75;
-            let est_tops = Tops { typo_ascent: est_asc, cap: est_asc * 0.72, xh: est_asc * 0.5 };
+            let est_tops = Tops { typo_ascent: est_asc, cap: est_asc * 0.72, xh: est_asc * 0.5, em: base_chars.size * 0.88 };
             let est_first = cur.next_baseline(f, col, base_leading, est_asc, est_tops, &pp);
             let est_jump = cur.jump(f, base_leading, est_asc, est_tops);
             let slots = estimate_slots(f, col, est_first, est_jump, base_leading, base_chars.size, &glyphs[g0..], &pp, line_no);
@@ -2093,25 +2093,34 @@ impl Cursor {
 fn first_baseline_offset(f: &FrameSpec, lead: f64, asc: f64, tops: Tops) -> f64 {
     let off = match f.opts.first_baseline {
         // Vertical frames measure from the ascent.
-        FirstBaseline::Ascent if f.vertical => asc,
+        FirstBaseline::Ascent | FirstBaseline::EmboxHeight if f.vertical => asc,
         FirstBaseline::CapHeight if f.vertical => asc * 0.72,
         FirstBaseline::XHeight if f.vertical => asc * 0.5,
         FirstBaseline::Ascent => tops.typo_ascent,
         FirstBaseline::CapHeight => tops.cap,
         FirstBaseline::XHeight => tops.xh,
+        FirstBaseline::EmboxHeight => tops.em,
         FirstBaseline::Leading => lead,
         FirstBaseline::Fixed => 0.0,
     };
     off.max(f.opts.first_baseline_min)
 }
 
-/// A line's tallest typographic ascender, cap height and x height above its baseline: the
+/// A line's tallest typographic ascender, cap height, x height and em box top above its baseline:
-/// first baseline offsets.
+/// the first baseline offsets.
 #[derive(Clone, Copy, Default)]
 struct Tops {
     typo_ascent: f64,
     cap: f64,
     xh: f64,
+    em: f64,
+}
+
+/// The em box top above the baseline at `size`: the font's `BASE` em box, else the default
+/// ideographic em box, 0.88 em above the baseline.
+fn em_box_top(face: &designcraft_fonts::FontFace, size: f64) -> f64 {
+    let em = face.declared_em_box().filter(|_| face.upem > 0.0).map_or(0.88, |(top, _)| top / face.upem);
+    em * size
 }
 
 /// Em box relative to the baseline. Font ascender/descender proportions locate it;
@@ -2160,7 +2169,7 @@ fn line_metrics(
     if src.is_empty() {
         let face = face();
         let k = base_size / face.upem;
-        let tops = Tops { typo_ascent: face.typo_ascent * k, cap: face.cap_height * k, xh: face.x_height * k };
+        let tops = Tops { typo_ascent: face.typo_ascent * k, cap: face.cap_height * k, xh: face.x_height * k, em: em_box_top(&face, base_size) };
         return (face.ascent * k, tops, face.descent * k, base_leading);
     }
     let mut asc: f64 = 0.0;
@@ -2177,6 +2186,7 @@ fn line_metrics(
         if !g.list_label {
             own_text = true;
             tops.typo_ascent = tops.typo_ascent.max(g.typo_ascent + up);
+            tops.em = tops.em.max(em_box_top(&g.face, g.size) + up);
         }
         tops.cap = tops.cap.max(g.cap + up);
         tops.xh = tops.xh.max(g.xh + up);
@@ -2187,6 +2197,7 @@ fn line_metrics(
         // Only a label (an empty list paragraph): as an empty paragraph.
         let face = face();
         tops.typo_ascent = face.typo_ascent * base_size / face.upem;
+        tops.em = em_box_top(&face, base_size);
     }
     (asc, tops, desc, lead)
 }

crates/compose/src/tests.rs

@@ -1695,6 +1695,47 @@ fn ascent_first_baseline_uses_the_typographic_ascender() {
     assert!(close(first(&d), top + 16.0), "{}", first(&d));
 }
 
+#[test]
+fn em_box_first_baseline_uses_the_base_em_box_else_0_88_em() {
+    use designcraft_doc::FirstBaseline;
+    let db = designcraft_fonts::FontDb::global();
+    // The test font's `BASE` table puts the em box bottom (`ideo`) at -170 units: its top is 0.83 em.
+    let mut no_base = typo_test_font("EmBoxTop 0700", Some(700));
+    let tables = u16::from_be_bytes([no_base[4], no_base[5]]) as usize;
+    let rec = (0..tables).map(|t| 12 + 16 * t).find(|&r| &no_base[r..r + 4] == b"BASE").unwrap();
+    // Still sorted between its neighbours, so the table directory stays valid.
+    no_base[rec..rec + 4].copy_from_slice(b"BASX");
+    db.add_font(no_base);
+    db.add_font(typo_test_font("EmBoxBase 700", Some(700)));
+    assert_eq!(db.face("EmBoxTop 0700", "Regular").declared_em_box(), None);
+    assert_eq!(db.face("EmBoxBase 700", "Regular").declared_em_box(), Some((830.0, -170.0)));
+    let (mut d, sid, fid) = doc_with("Hxg Hxg", Rect::new(36.0, 36.0, 300.0, 300.0), ParaAttrs::default());
+    let mut first = |family: &str, sizes: [f64; 2], kind: FirstBaseline| {
+        let st = d.story_mut(sid).unwrap();
+        for (range, size) in [(0..4, sizes[0]), (4..7, sizes[1])] {
+            st.format_chars(range, |f| {
+                f.over.font_family = Some(family.into());
+                f.over.font_style = Some("Regular".into());
+                f.over.size = Some(size);
+            });
+        }
+        d.item_mut(fid).unwrap().text_frame_mut().unwrap().options.first_baseline = kind;
+        compose_story(&d, sid, &ComposeOptions::default()).frames[0].lines[0].baseline - 36.0
+    };
+    let close = |a: f64, b: f64| (a - b).abs() < 0.01;
+    // Ascent: the 0.7 em typographic ascender; Em Box Height: the 0.88 em default em box top.
+    let asc = first("EmBoxTop 0700", [10.0; 2], FirstBaseline::Ascent);
+    assert!(close(asc, 7.0), "{asc}");
+    let emb = first("EmBoxTop 0700", [10.0; 2], FirstBaseline::EmboxHeight);
+    assert!(close(emb, 8.8), "{emb}");
+    // A mixed first line takes its largest size.
+    let emb = first("EmBoxTop 0700", [10.0, 20.0], FirstBaseline::EmboxHeight);
+    assert!(close(emb, 17.6), "{emb}");
+    // A font's `BASE` em box wins over the default.
+    let emb = first("EmBoxBase 700", [10.0; 2], FirstBaseline::EmboxHeight);
+    assert!(close(emb, 8.3), "{emb}");
+}
+
 #[test]
 fn ascent_first_baseline_falls_back_to_the_ascent_without_a_usable_typo_ascender() {
     let db = designcraft_fonts::FontDb::global();

crates/doc/src/item.rs

@@ -281,6 +281,8 @@ pub enum FirstBaseline {
     Leading,
     XHeight,
     Fixed,
+    /// The top of the em box: the font's `BASE` em box, else 0.88 em above the baseline.
+    EmboxHeight,
 }
 
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]

crates/engine/src/cmd/notes.rs

@@ -22,7 +22,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Document Footnote Options…",
             ["Type"],
             None,
-            "{style?: arabic|upperRoman|lowerRoman|upperLetters|lowerLetters|symbols, startAt?, restart?: never|page|spread|section, prefix?, suffix?, affixIn?: none|reference|text|both, refPosition?: superscript|subscript|normal|otSuperscript, refCharStyle?, paraStyle?, separator?, spaceBefore?, spaceBetween?, firstBaseline?, firstBaselineMin?, spanColumns?, rule?: {on, weight, color, tint, width, offset, leftIndent}} → the options (no params: just read them)",
+            "{style?: arabic|upperRoman|lowerRoman|upperLetters|lowerLetters|symbols, startAt?, restart?: never|page|spread|section, prefix?, suffix?, affixIn?: none|reference|text|both, refPosition?: superscript|subscript|normal|otSuperscript, refCharStyle?, paraStyle?, separator?, spaceBefore?, spaceBetween?, firstBaseline?: ascent|capHeight|leading|xHeight|emboxHeight|fixed, firstBaselineMin?, spanColumns?, rule?: {on, weight, color, tint, width, offset, leftIndent}} → the options (no params: just read them)",
             has_doc,
             options
         ),

crates/engine/src/cmd/object.rs

@@ -489,7 +489,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Text Frame Options…",
             ["Object"],
             Some("Cmd+B"),
-            "{columns?, gutter?, inset?: number|[t,l,b,r] (null keeps that side), verticalJustification?: top|center|bottom|justify, firstBaseline?, autoSize?, ignoreWrap?, balanceColumns?, columnRule?: bool, columnRuleWeight? (pt), columnRuleColor? (swatch name), columnRuleTint? (0..1), columnRuleOffset? (pt, horizontal), columnRuleTopInset? (pt), columnRuleBottomInset? (pt), vertical?: bool (sets the story direction of the frames' stories, as Type ▸ Story Direction), ids?}",
+            "{columns?, gutter?, inset?: number|[t,l,b,r] (null keeps that side), verticalJustification?: top|center|bottom|justify, firstBaseline?: ascent|capHeight|leading|xHeight|emboxHeight|fixed, autoSize?, ignoreWrap?, balanceColumns?, columnRule?: bool, columnRuleWeight? (pt), columnRuleColor? (swatch name), columnRuleTint? (0..1), columnRuleOffset? (pt, horizontal), columnRuleTopInset? (pt), columnRuleBottomInset? (pt), vertical?: bool (sets the story direction of the frames' stories, as Type ▸ Story Direction), ids?}",
             has_selection,
             text_frame_options
         ),

crates/fonts/src/fontdb.rs

@@ -122,7 +122,7 @@ pub struct FontFace {
     /// Basic Multilingual Plane coverage bitset, built on first use.
     bmp: std::sync::OnceLock<Box<[u64]>>,
     /// The ideographic em box, read on first use.
-    em: std::sync::OnceLock<(f64, f64)>,
+    em: std::sync::OnceLock<((f64, f64), bool)>,
     /// The font menu group and native family name, read on first use.
     group: std::sync::OnceLock<(FontGroup, Option<String>)>,
 }
@@ -267,7 +267,12 @@ impl FontFace {
     /// -120): `BASE` `idtp`/`ideo`, else the OS/2 typo ascender/descender, else the ascender and
     /// descender, the last two centred on one em.
     pub fn em_box(&self) -> (f64, f64) {
-        *self.em.get_or_init(|| crate::vertical::em_box(self))
+        self.em.get_or_init(|| crate::vertical::em_box(self)).0
+    }
+    /// The em box (top, bottom) in font units, y up, when the font declares one in `BASE`.
+    pub fn declared_em_box(&self) -> Option<(f64, f64)> {
+        let (b, declared) = *self.em.get_or_init(|| crate::vertical::em_box(self));
+        declared.then_some(b)
     }
     /// The font menu group the font is for and, for a CJK group, its family name in that language
     /// (see [`crate::FamilyInfo`]); read from the font once.

crates/fonts/src/vertical.rs

@@ -55,11 +55,12 @@ impl<'a> VMetrics<'a> {
 
 /// The ideographic em box (top, bottom) in font units, y up: `BASE` `idtp`/`ideo`, else the OS/2
 /// typo ascender and descender, else the ascender and descender; the last two centred on one em.
-pub(crate) fn em_box(face: &FontFace) -> (f64, f64) {
+/// The flag is true for an em box from `BASE`.
+pub(crate) fn em_box(face: &FontFace) -> ((f64, f64), bool) {
     let upem = face.upem;
     let font = face.skrifa();
     if let Some(b) = font.as_ref().and_then(|f| base_em_box(f, upem)) {
-        return b;
+        return (b, true);
     }
     let centred = |asc: f64, desc: f64| {
         let span = asc - desc;
@@ -71,7 +72,7 @@ pub(crate) fn em_box(face: &FontFace) -> (f64, f64) {
     font.and_then(|f| f.os2().ok())
         .and_then(|t| centred(f64::from(t.s_typo_ascender()), f64::from(t.s_typo_descender())))
         .or_else(|| centred(face.ascent, -face.descent))
-        .unwrap_or((upem * 0.88, upem * -0.12))
+        .map_or(((upem * 0.88, upem * -0.12), false), |b| (b, false))
 }
 
 /// The em box from the `BASE` table's horizontal axis: the Han script's values, else the default

crates/idml/src/names.rs

@@ -424,10 +424,11 @@ pub fn first_baseline_out(f: FirstBaseline) -> &'static str {
         FirstBaseline::Leading => "LeadingOffset",
         FirstBaseline::XHeight => "XHeight",
         FirstBaseline::Fixed => "FixedHeight",
+        FirstBaseline::EmboxHeight => "EmboxHeight",
     }
 }
 pub fn first_baseline_in(s: &str) -> FirstBaseline {
-    [FirstBaseline::CapHeight, FirstBaseline::Leading, FirstBaseline::XHeight, FirstBaseline::Fixed]
+    [FirstBaseline::CapHeight, FirstBaseline::Leading, FirstBaseline::XHeight, FirstBaseline::Fixed, FirstBaseline::EmboxHeight]
         .into_iter()
         .find(|m| first_baseline_out(*m) == s)
         .unwrap_or(FirstBaseline::Ascent)

crates/idml/src/tests.rs

@@ -378,6 +378,17 @@ fn imports_scalar_inset_properties_like_lists_and_attributes() {
     assert_eq!(frame_inset(&round_trip), [9.0; 4]);
 }
 
+#[test]
+fn em_box_first_baseline_offset_round_trips() {
+    use designcraft_doc::FirstBaseline;
+    let first = |d: &Document| d.spreads[0].items[0].text_frame().unwrap().options.first_baseline;
+    let d = inset_fixture(r#"<TextFramePreference FirstBaselineOffset="EmboxHeight"/>"#, "");
+    assert_eq!(first(&d), FirstBaseline::EmboxHeight);
+    assert_eq!(first(&import_idml(&export_idml(&d)).unwrap()), FirstBaseline::EmboxHeight);
+    let unknown = inset_fixture(r#"<TextFramePreference FirstBaselineOffset="SomethingElse"/>"#, "");
+    assert_eq!(first(&unknown), FirstBaseline::Ascent);
+}
+
 #[test]
 fn inset_property_keeps_precedence_over_attribute() {
     for (property, expected) in [

crates/ui-egui/src/dialogs.rs

@@ -3373,7 +3373,14 @@ fn footnote_options(app: &mut DesignApp, ui: &mut egui::Ui, d: &mut Dialog) {
                 ui,
                 d,
                 "firstBaseline",
-                &[("ascent", "Ascent"), ("capHeight", "Cap Height"), ("leading", "Leading"), ("xHeight", "x Height"), ("fixed", "Fixed")],
+                &[
+                    ("ascent", "Ascent"),
+                    ("capHeight", "Cap Height"),
+                    ("leading", "Leading"),
+                    ("xHeight", "x Height"),
+                    ("emboxHeight", "Em Box Height"),
+                    ("fixed", "Fixed"),
+                ],
             );
             ui.end_row();
             crate::rtl::label(ui, crate::i18n::tr(&app.ui.language, "Min:"));

crates/ui-egui/src/i18n.rs

@@ -1941,6 +1941,7 @@ const TABLE: &[(&str, [&str; 5])] = &[
     ("Cap Height", ["Cap Height", "Cap Height", "Cap Height", "Cap Height", "大写字母高度"]),
     ("Leading", ["Leading", "Leading", "Leading", "Leading", "行距"]),
     ("x Height", ["x Height", "x Height", "x Height", "x Height", "x 字高"]),
+    ("Em Box Height", ["Em Box Height", "Em Box Height", "Em Box Height", "Em Box Height", "全角字框高度"]),
     ("Fixed", ["Fixed", "Fixed", "Fixed", "Fixed", "固定"]),
     ("Never (continuous)", ["Never (continuous)", "Never (continuous)", "Never (continuous)", "Never (continuous)", "从不（连续编号）"]),
     ("Page", ["Page", "Page", "Page", "Page", "页面"]),

crates/ui-egui/src/i18n/ar.rs

@@ -296,6 +296,7 @@ pub(super) const TABLE: &[(&str, &str)] = &[
     ("Ascent", "الصعود"),
     ("Cap Height", "ارتفاع الأحرف الكبيرة"),
     ("x Height", "ارتفاع x"),
+    ("Em Box Height", "ارتفاع مربع em"),
     ("Fixed", "ثابت"),
     ("Never (continuous)", "أبدًا (ترقيم متصل)"),
     ("Spread", "صفحتان متقابلتان"),

crates/ui-egui/src/i18n/it.rs

@@ -961,6 +961,7 @@ pub(super) const TABLE: &[(&str, &str)] = &[
     ("Cap Height", "Altezza maiuscole"),
     ("Leading", "Interlinea"),
     ("x Height", "Altezza x"),
+    ("Em Box Height", "Altezza riquadro em"),
     ("Fixed", "Fisso"),
     ("Never (continuous)", "Mai (continua)"),
     ("Page", "Pagina"),

crates/ui-egui/src/i18n/ja.rs

@@ -960,6 +960,7 @@ pub(super) const TABLE: &[(&str, &str)] = &[
     ("Cap Height", "大文字の高さ"),
     ("Leading", "行送り"),
     ("x Height", "x の高さ"),
+    ("Em Box Height", "仮想ボディの高さ"),
     ("Fixed", "固定"),
     ("Never (continuous)", "振り直しなし (通し番号)"),
     ("Page", "ページ"),

crates/ui-egui/src/i18n/pt_br.rs

@@ -297,6 +297,7 @@ pub(super) const TABLE: &[(&str, &str)] = &[
     ("Ascent", "Ascendente"),
     ("Cap Height", "Altura da caixa alta"),
     ("x Height", "Altura do x"),
+    ("Em Box Height", "Altura da caixa eme"),
     ("Fixed", "Fixo"),
     ("Never (continuous)", "Nunca (contínuo)"),
     ("Spread", "Página dupla"),

crates/ui-egui/src/i18n/uk.rs

@@ -958,6 +958,7 @@ pub(super) const TABLE: &[(&str, &str)] = &[
     ("Cap Height", "Висота великих літер"),
     ("Leading", "Міжрядковий інтервал"),
     ("x Height", "Висота малих літер"),
+    ("Em Box Height", "Висота em-квадрата"),
     ("Fixed", "Фіксований"),
     ("Never (continuous)", "Ніколи (наскрізна нумерація)"),
     ("Page", "Сторінка"),